The invention relates to a method and an arrangement for operating an internal combustion engine, especially of a charged internal combustion engine.
Engines having exhaust-gas turbocharging display, as a rule, a torque weakness during start-to-drive. This applies especially to such engines without a variable geometry of the turbine of the exhaust-gas turbocharger. The reason for this is that the small exhaust-gas mass flow of the engine at a start-to-drive rpm leads to very poor rates of efficiency of the turbine as well as of the compressor of the exhaust-gas turbocharger. This results in a weak charge pressure buildup at low torque in the lower rpm range of the engine. In very extreme situations, for example, in steep mountain pass travel at high elevation with switched-in ancillary apparatus, a start-to-drive operation can be considerably impeded. German patent publication 199 44 190 discloses measures for increasing the efficiency of an exhaust-gas turbocharger. This is achieved with an after-injection which is carried out after the primary injection. With this after-injection, the energy content of the exhaust gas (especially the exhaust-gas enthalpy for the turbine of the exhaust-gas turbocharger) is persistently increased so that the response performance of the engine is significantly improved with a torque or power request as a consequence of the increased charge pressure. In this correction, it has been shown to be practical to so control the inlet and outlet valves of the cylinders during the after-injection so that there is a greater valve overlapping. In this way, a greater air throughput occurs at the turbine and a greater pressure occurs on the turbine so that a higher charge pressure can be built up. The after-injection is so undertaken that a combustible mixture results because of the air excess present in the individual combustion chambers and this combustible mixture then ignites as a consequence of the still running combustion and/or the high temperature. The generation of an air excess in the cylinders, in which the after-injection takes place, is likewise provided.
Another possibility of achieving an increase of the charge pressure in the lower rpm range is the use of an electrical turbocompressor, preferably a turbocompressor in the form of an ancillary charger to the exhaust-gas turbocharger. The electrical turbocompressor is driven in the low rpm range by driving its electric motor to increase the charge pressure. A large valve overlapment is pregiven either because of construction or is generated via an inlet camshaft shift and/or an outlet camshaft shift. In combination with the large valve overlapment and the large pressure drop between the intake manifold and the exhaust-gas system ahead of the turbine of the exhaust-gas turbocharger, a high scavenging air-mass flow results. This type of charged internal combustion engine having the possibility of an after-injection, which does not participate in the internal combustion process, has a considerable potential for a power increase in the described range.
There is also a need for a solution to the exhaust-gas enthalpy increase in charged internal combustion engines wherein an after-injection (which does not participate in the internal combustion process) is provided, preferably in combination with an externally driven electrical charger.
A significantly higher enthalpy drop across the turbine of the exhaust-gas turbocharger is generated by a combination of an after-injection with an external ignition assist which is placed upstream of the turbine of the exhaust-gas turbocharger in the exhaust-gas system. The after-injection does not participate in the internal combustion process.
In an advantageous manner, by the increase of the exhaust-gas enthalpy drop across the turbine of the exhaust-gas turbocharger, the start-drive performance of the vehicle is significantly improved as a consequence of the increased charge pressure.
A further advantage results with a combination of the exhaust-gas turbocharger with an additional ancillary charger, for example, an electrically driven charger which generates high scavenging air-mass flows through the internal combustion engine. This advantage is increased by an additional camshaft displacement which contributes to a larger valve overlapment and therefore to a further increase of the scavenging air-mass flow. The reason for this is the pressure drop, which is present between the intake manifold system and the exhaust-gas system. This pressure drop is significantly increased by the electrical ancillary charger. This pressure drop in combination with variable valve control times is used during the valve overlapment phase about top dead center for large scavenging air-mass flows during charge change (up to approximately 30% of the work air-mass flow). If, at the same time, an injection of fuel takes place, then an ignitable air/fuel mixture is formed which reliably ignites because of an ignition aid mounted upstream of the turbine of the exhaust-gas turbocharger and is combusted. In this way, the useful enthalpy drop increases significantly at the turbine of the exhaust-gas turbocharger and increases the available pressure at the compressor end.
In principle, the above-mentioned advantages are also obtained in charged internal combustion engines without an externally driven ancillary charger with an after-injection and a subsequent ignition in the exhaust-gas system.
It is also especially advantageous that the charger system can be transferred into a self-maintaining state via a suitable design of the components. In this way, after switchoff of the electrical ancillary charger of the exhaust-gas turbocharger, in combination with the additional enthalpy availability from the post-combustion of the scavenged air mass, the charge pressure increase can be maintained with this pressure charging increase being obtained by the additional charger.
The consequence is an operating point of favorable efficiency of the exhaust-gas turbocharger via the increased exhaust-gas mass and the increased enthalpy.
A further advantage of the described procedure is the fact that the additional ignition aid in combination with secondary air can be used for a more rapid heating of the catalytic converter to operating temperatures.